1. Field of the Invention
This invention relates to an automatic tracking apparatus for use in a video tape recorder (VTR) including a magnetic head mounted on a piezo-electric element and, more particularly, to an automatic tracking apparatus including a driving pulse generator for generating a driving pulse signal to deflect the piezo-electric element so as to drive the magnetic head.
2. Description of the Prior Art
In video tape recorders (VTR) using a rotary magnetic head mounted on a piezo-electric element, such as a bimorph element, an automatic tracking apparatus generates a driving pulse signal at a predetermined frequency to deflect the piezo-electric element so as to drive the magnetic head in reproducing a video signal. The reproduced video signal is applied to an envelope detector circuit which generates an envelope signal corresponding to the envelope of the reproduced video signal. The automatic tracking apparatus controls the deflection of the piezo-electric element based upon the envelope signal so as to cause the magnetic head to accurately follow a desired recording track in a changed-speed reproduction mode. Such a conventional automatic tracking apparatus is disclosed, for example, in Japanese Patent Publication No. 58-27866.
In such conventional automatic tracking apparatus, when the magnetic head jumps from one recording track to another recording track, the piezo-electric element is subjected to its maximum deflection and thus maximum mechanical vibrations (referred hereinafter to as ringing vibrations), causing tracking error. It is necessary to suppress ringing vibrations as much as possible.
In order to suppress the ringing vibrations which occur when the magnetic head jumps, an attempt has been made to provide an inflection point in the waveform of the driving pulse signal to the piezo-electric element during the magnetic head jumping period so that the driving pulse signal increases at a smaller rate in the latter stage of the magnetic head jumping period, thereby suppressing the ringing vibrations, as disclosed, for example, in Japanese Patent Application Nos. 59-13910 and 59-13911, assigned to the assignee of the present invention.
Such an automatic tracking apparatus is satisfactory for general application, but it has been found that its usefulness is limited in specialized applications, particularly where the magnetic head jumps over two or three recording tracks to a demanded recording track in a changed-speed reproduction mode. During this operation, the piezo-electric element is subject to much greater deflection and thus much greater ringing vibrations. Both accurate tracking control and suppression of ringing vibrations below a desired level cannot be expected without setting the inflection point with much greater accuracy. In addition, the characteristics of the piezo-electric element change with time and environmental conditions, causing deviation of the inflection point from the desired position. It is impossible for a user to readjust the inflection point to the desired position.
In order to eliminate the problems associated with conventional tracking apparatus, the present invention provides an improved automatic tracking apparatus which can provide superior tracking servo control ensuring that the inflection point is automatically adjusted to an optimum position in the waveform of a driving pulse signal sent to the piezo-electric element.